1. Field of Invention
The invention relates to a light emitting diode (LED) driving circuit, a LED driving device and a LED driving method.
2. Related Art
Because the light emitting diode (LED) has the higher opto-electronic conversion efficiency and the high operation stability and has the luminance that can be controlled (also referred to as the gray scale control) by way of pulse width modulation (PWM), the LEDs have been applied to various light sources or display elements in various electronic devices, such as a backlight module of a display device, an illumination device, an advertising billboard or image pixels of a large-scale display device.
Referring to FIG. 1A, a conventional LED driving circuit 1 has a data register unit 11, a counter 12, a comparator 13 and a driver 14. The data register unit 11 receives and stores the gray scale information coming from the system end (not shown). The counter 12 receives the clock signal outputted from the system end. The comparator 13 has a first end 131 coupled to the data register unit 11, and a second end 132 coupled to the counter 12. The first end 131 and the second end 132 receive the signals outputted from the data register unit 11 and the counter 12, respectively. The comparator 13 compares the signals, received by the first end 131 and the second end 132, with each other. When the signal received by the first end 131 is higher than the signal received by the second end 132, the output terminal of the comparator 13 has a logic high potential, so that the driver 14 lights up the LED with a constant current source. When the signal received by the second end 132 is higher than the signal received by the first end 131, the output terminal of the comparator 13 has the logic low potential. At this time, the driver 14 does not light up the LED. Therefore, as shown in FIG. 1B, the driver 14 outputs a PWM signal in accordance with the comparison result of the comparator 13, so that the LED generates the luminance with different gray scales, and a turn-on time interval T1 of the PWM signal in a duty cycle T is a continuous turn-on time interval. The gray scale represents the brightness/darkness level of the luminance, and the conventional LED driving circuit 1 makes the LED to output different luminance levels through the PWM signal outputted from the driver 14. As the turn-on time interval T1 gets longer, the time for lighting the LED gets longer and the luminance gets brighter. On the contrary, as the turn-on time interval T1 gets shorter, the luminance becomes darker. When the turn-on time interval T1 is zero, it represents that the LED is extinguished.
In order to compare the signals, coming from the data register unit 11 and the counter 12, with each other, however, the comparator 13 of the conventional LED driving circuit 1 is composed of a lot of metal-oxide-semiconductor field-effect transistors (MOSFETs). For example, when the comparator 13 uses a 12-bit comparator, the comparator 13 has at least 864 MOSFETs. Because the MOSFET itself has the defects of the leakage current and the parasitic capacitance, the comparator 13 using a lot of MOSFETs has the problem of the additional power loss.
It is an important subject to provide a LED driving circuit, capable of reducing unessential loss of the power and enhancing the processing performance, and a LED driving device and a LED driving method.